my-module.nix

{ config, pkgs, lib, ... }:

let
  cfg = config.installHello;
in
{
  options = {
    installHello.enable = lib.mkEnableOption "Some way to install GNU hello";
  };

  config = lib.mkIf cfg.enable {
    environment.systemPackages = [ pkgs.hello ];
  };
}

options.nix

let
  pkgs = import <nixpkgs> { };

  eval = import <nixpkgs/nixos/lib/eval-config.nix> {
    system = builtins.currentSystem;
    modules = [
      {
        # got this from https://github.com/NixOS/mobile-nixos/blob/3baf77089e7793f9322d7b28710f43ed5f8b886d/doc/_support/options/default.nix#L25-L39
        disabledModules = [
          <nixpkgs/nixos/modules/services/networking/blockbook-frontend.nix>
        ];
      }
    ];
    extraModules = [ ./my-module.nix ];
  };

  optionsDoc = pkgs.nixosOptionsDoc { options = eval.options; };
in
  optionsDoc.optionsJSON


# not using that disabledModules line results in:
#$ nix-build options.nix
#error: Package ‘blockbook-0.3.4’ in /nix/store/zqnsghhc12321212h5hvsrvclc4g8aci-nixos-21.05.650.eaba7870ffc/nixos/pkgs/servers/blockbook/default.nix:63 is marked as broken, refusing to evaluate.
#
#       a) To temporarily allow broken packages, you can use an environment variable
#          for a single invocation of the nix tools.
#
#            $ export NIXPKGS_ALLOW_BROKEN=1
#
#       b) For `nixos-rebuild` you can set
#         { nixpkgs.config.allowBroken = true; }
#       in configuration.nix to override this.
#
#       c) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
#         { allowBroken = true; }
#       to ~/.config/nixpkgs/config.nix.
#(use '--show-trace' to show detailed location information)